1. Field of the Invention
The present invention relates, in general, to power systems and devices, and more particularly to a technique for regulating the output voltage of a DC-DC power converter. Specifically, the invention is concerned with modulating the duty cycle of a self-oscillating DC-DC power converter by controlling the magnetic flux reset time of the power transformer core. The present invention is particularly, but not exclusively applicable to low-power DC-DC power converters.
2. Description of the Related Technology
Self-oscillating regulators and power converters are old and well known in the prior art as exemplified by U.S. Pat. Nos. 5,012,399, 4,443,838, and 4,605,999. It is well known that typical self-oscillating power converters operate by storing and releasing energy in various discrete capacitive and inductive components during each cycle of operation. The operation cycles of the power converter are controlled by the switching frequency of an input power switching device. In conventional DC-DC converters, the input power switch is typically a semiconductor switch which turns on and off repetitively to provide output voltage conversion and output voltage regulation.
It is also well known to use control circuits to control the duty cycle of the input power switch which controls energization of the primary winding of a power transformer, which in turn regulates the output of the switching power converter. The control circuits utilized for controlling the duty cycle of a power transformer of the prior art are exemplified by U.S. Pat. Nos. 3,989,995 and 4,926,303. In such DC-DC converters, a direct current input is switched across the primary of an isolation transformer, and a secondary voltage is rectified and filtered to provide a direct current output.
Royer and Jensen converters are well known in the art. It is well known that many low cost, self-oscillating circuits, such as the "flyback", Royer and Jensen converters perform this function. However, because these converters deliver power to the output when the input power switch turns off, or rely on a magnetic element that saturates, simple means of control cannot be easily implemented. In fact, substantial additional components are required, including feedback networks, isolation components, and regulator circuits. The number of additional components required for output regulation in converters of the prior art often exceed the number of components required for simple unregulated power conversion. Furthermore, the operational characteristics of the prior art converters place stress on circuit components and create relatively low efficiency ratings. As is well known, cost and size is greatly influenced by the number of parts needed to construct the control circuit.
It is also well known that self-oscillating converters of the prior art operate with relatively large ripple currents or large current peaks on their input and output capacitors. In addition, in the prior art converters, the transformer primary, the input power switch, and the output section typically experience large current peaks. Large current peaks detrimentally affects the performance of the converters of the prior art.
For example, the Royer and Jensen converters use magnetic elements to deliver power to their outputs. When the magnetic elements saturate, large current peaks are produced on the input capacitors. The Royer and Jensen converters produce even greater current peaks at start-up (the initial application of input voltage) due to discontinuous inrush currents charging their output capacitors. Similarly, the flyback converters produce discontinuous currents on their outputs causing large current peaks to be produced on their output capacitors. The larger current peaks lead to lower circuit efficiency.
Control circuits which implement the output regulation of prior art self-oscillating converters are also well known in the art. The purpose of output regulation is to provide a regulated output which remains constant over varying input voltages and varying output loading conditions. Typically, especially in the case of the flyback converter, information relating to the output voltage is fed back to the self-oscillating primary circuit where it is used to control the duty cycle of the input power switch. Regulation of the Royer and Jensen outputs is accomplished by pre-regulating the voltage realized by the self-oscillating primary circuit. The Royer and Jensen circuits use feedback information from the output voltage to control the pre-regulation circuits. However, due to cost constraints, most Royer and Jensen circuits do not provide output regulation. Furthermore, in all of the converters of the prior art, the output-to-primary feedback regulation technique leads to poor input to output isolation and adversely affects converter stability.
As a result, control circuits used for controlling the duty cycle of the prior art DC-DC converters require isolating elements, for example, optical couplers, in order to couple information relating to the output of the secondary side of the transformer to the primary side. Such circuitry is costly and requires numerous circuit elements in order to perform the control operation.
Control circuits of the prior art, used for controlling the duty cycle of power converters, have also utilized control transformers for regulation. The use of a transformer to control the on-time and off-time of the input power switch, introduces added cost and decreased efficiency into the power converter system design. Furthermore, it is also well known to use magnetic elements to regulate the output voltage. Magnetic elements are used in the prior art converters to limit the volt-seconds applied to the power transformer (the product of the voltage multiplied by the time that the voltage is applied to the power transformer). After blocking the volt-seconds applied to the power transformer, the magnetic elements then saturate. By controlling the saturation delay of the magnetic elements, the converters of the prior art regulate the output voltage. However, using such magnetic elements for output regulation reduces converter efficiency and requires additional flux reset timing control.
Therefore, the need has arisen for a DC-DC power converter that has output regulation, increased reliability, increased stability, increased input to output isolation, operates over a wide range of input voltages, is physically small in size, and is implemented at low cost with few components.